A typical satellite antenna system comprises a reflector dish and a horn antenna to convey radio waves between a transmitter/receiver (transceiver) and the reflector dish. In satellite communications the phenomenon of squint is well known. When the feed horn illuminates the parabolic offset reflector dish with a circularly polarized wave as shown in FIG. 1, the main lobe of the reflected secondary farfield radiation pattern is shifted away from the boresight axis (i.e. the axis of symmetry of the paraboloid defining the reflector dish) of the reflector dish in the horizontal plane. The direction in which the main lobe deviates is determined by the polarization: for the offset configuration typically used, left handed circularly polarized (LHCP) waves shift to the left and right handed circularly polarized (RHCP) waves shift to the right. The squint angle is inversely proportional to frequency. For a typical Ka-Ka band antenna configuration, the squint angle is around 60 mdeg in the transmit (TX) band (30 GHz) and around 90 mdeg in the receive (RX) band (20 GHz). Note that the Ka band satellite terminals typically use transmit frequencies between 27.5 GHz and 31 GHz and receive frequencies between 18.3 and 22 GHz. This large ratio between the transmit and receive frequencies (as compared to the frequency span of e.g. the Ku-band frequency plan (10.7-14.5 GHz)) imposes a challenge in the implementation of an antenna feed system.
Squint mainly causes problems when the receive and transmit beams are squinted in opposite directions, as occurs in a cross-polarized configuration, where the receive and transmit signals have opposite polarization. Such a situation is illustrated in FIG. 2: the RHCP transmit beam is shifted by 60 mdeg, the LHCP receive beam is shifted by −90 mdeg. This graph shows that when the antenna is pointed towards the satellite based on the received signal power, a minor receiver pointing error of 0.5 dB may cause a worst case pointing loss of 2.5 dB on the transmit channel.
Some more background information on the origin of squint is now provided. A parabolic offset reflector dish is considered illuminated by a linearly polarized feed horn that does not generate cross-polarized components when illuminating a prime focus reflector. The aperture fields of the reflector are depicted in FIG. 3 for linear horizontal and vertical polarizations, respectively. The presence of cross-polar field components is apparent: an offset reflector dish fed by a linearly polarized feed causes cross-polarization, even though the same feed will not create cross-polarization when used for a prime focus reflector.
The result for a circularly polarized horn is different and can be explained by considering that for both linear polarizations components of the circular polarized wave, the E-fields to the left side of the boresight axis are tilted counterclockwise, the E-fields to the right side clockwise. This means that for RHCP, the fields to the right are lagging in phase, the fields to the left are leading (and the opposite for LHCP). This phase difference causes the main lobe to shift away from the boresight axis, thus causing squint. The squint angle is given by the following equation
      sin    ⁢                  ⁢    θ    =                    ±        sin            ⁢                          ⁢      ψ              4      ⁢                          ⁢      π      ⁢              F        λ            whereby θ is the squint angle, ψ the offset angle, F the focal length and λ the wavelength.
While the end result is different for linear and circular polarization, the underlying cause is exactly the same. Techniques used to cancel cross-polar radiation for linear polarization are thus also suitable for compensating the squint in the case of circular polarization, provided that the phase relationship between the vertical and horizontal components remains undisturbed by the applied compensation technique.
FIG. 3(b) depicts the aperture fields of an offset reflector dish illuminated by a vertically polarized feed. The presence of horizontal field components is clear. Furthermore, they are symmetric with respect to the vertical axis. It is well known in the art that these components could be compensated by adding a waveguide mode with a field pattern where the horizontal field components are also symmetric with respect to the vertical axis. The vertically polarized TE21 mode has this property, as illustrated in FIG. 4(a). It is thus possible to cancel the cross-polar components by adding this TE21 mode with the right amplitude and phase to the fundamental mode (i.e. the TE11 mode).
The case of horizontal polarization is similar. The aperture fields of the offset reflector are shown in FIG. 3(a). The presence of vertical field components, anti-symmetric with respect to the vertical axis, is also here apparent. They can be cancelled by adding a waveguide mode with a field pattern where the vertical components are anti-symmetric with respect to the vertical axis. The horizontally polarized TE21 mode has this property, as is shown in FIG. 4(b). For the horizontal polarization, it is thus again possible to cancel the cross-polar components by adding this TE21 mode with the right amplitude and phase to the fundamental mode (i.e. the TE11 mode).
The above analysis was made for the compensation of cross-polar components in the case of linear polarization. One extra condition for squint compensation in case of circular polarization concerns the fact that the phase relationship between the vertically and horizontally polarized components must be maintained at 90°.
An antenna feed system which cancels cross-polar components for linear polarization and which maintains the correct phase relationship between both linear polarisations compensates squint so that in the frequency band where compensation is applied the beams for RHCP and LHCP are both aligned with the boresight axis. However, in case of opposite polarized transmit and receive beams, and if squint compensation is only implemented at the transmit frequency band, then the beam of transmit (e.g. RHCP) will not align with the receive beam (e.g. LHCP), because the latter is unaffected by squint compensation, and hence will not align along the boresight axis. Therefore, in this case, a solution would be desirable to overcompensate the squint and have the transmit beam aligned with the receive beam rather than with the boresight axis.
In the prior art several solutions have been proposed for squint compensation (for circular polarization) and for cross-polar cancellation (for linear polarization). The paper “Dual-polarised mode generator for cross polar compensation in offset parabolic reflector antennas” (Watson et al., European Microwave Conference Proceedings, Paris, 1979, pp. 183-187) proposes a horn based on the Potter-horn principle, which is extended with a mode generator for compensating cross-polar components for horizontal and vertical linear polarization. The mode generator is based on adding three rectangular waveguide stubs at 0° and ±45° which generate the TE21 modes. Antecedent waveguide sections, i.e. located further away from the feed horn antenna, have a smaller diameter so that in this region, any generated TE21 mode is in cut-off and therefore cannot propagate. Hence, any TE21 mode generated further on but propagating backward will thus reflect at the smaller diameter step and combine with the forward propagating TE21 modes. This is a way of isolating the TE21 modes from any antecedent waveguide structure, such as a polarizer or an OMT (orthomode transducer). This is illustrated in FIG. 5. However, in a Ka-band architecture with RX frequency band 18.3-22 GHz and TX frequency band 27.5-31 GHz, for the TE21 mode to be in cut-off for both bands, the diameter would need to be smaller than 9.4 mm, but this in turn would imply that the cut-off frequencies of the TE11 modes are around 18.7 GHz, which is in conflict with the receive frequency range. Therefore, having a smaller diameter waveguide section which keeps the TE21 modes in cut-off in the transmit-band is not a viable option for the Ka band frequency plan. Furthermore, the three stubs in this arrangement may upset the phase relationship between the horizontally and vertically polarized TE11 modes, causing high cross-polarization in the radiated patterns when using this horn concept for circular polarization.
A variant of the feed horn proposed by Watson is disclosed in patent document EP 1278266 B1, where grooves are integrated in the step so that the component can be manufactured by die casting. The variant solution has been applied in a Ku-band (receive 10.7-12.7 GHz, transmit 14-14.5 GHz) product. The main characteristics of the proposed feed horn are similar to Watson. The step in diameter yields a waveguide section where the TE21 modes cannot propagate. As described above, this technique cannot be scaled to a Ka band frequency plan. This horn compensates the cross-polar components when using linear polarization, but, again, is not optimized for circular polarization.
Also in GB 1525514 a section of narrow waveguide blocks backward propagating TE21 modes.
Sharma et al. describe in “Removal of Beam Squinting Effects in a Circularly Polarized Offset Parabolic Reflector Antenna Using a Matched Feed” (Progress in Electromagnetics Research Letters, vol. 7, pp. 105-114, 2009) a tri-mode matched feed horn to remove the beam squinting effects in a circularly polarized offset parabolic reflector antenna. The solution is based on three pins, which are spaced apart over 120°.